Hitherto, oil country tubular goods (steel pipes to be used at oil wells) are connected to each other by a joint. A coupling that constructs the joint has formed therein female threads (sometimes referred to as “box threads”) gradually expanded from both ends to a center of the coupling, whereas the steel pipe (hereinafter sometimes referred to as “pin”) has male threads (sometimes referred to as “pin threads”) formed at an end portion of the steel pipe to screw in the female threads.
The structure in which the female threads and the male threads are screwed in each other to connect the pair of steel pipes to each other by the joint is hereinafter referred to as “joint structure”.
Further, the joint structure is required to have gas-tightness and anti-looseness. From the viewpoint of enhancing the gas-tightness and the anti-looseness, there is disclosed a threaded joint for steel pipes, in which the shapes of the female threads and the male threads are limited (see, for example, Patent Literature 1).